- CO2 ranged 180 - 4000 ppm in last 500 million years
- CO2 changes impact living things via photosynthesis
- primary source of energy in food chain
- large evolutionary selection pressure
- Large decline in CO2 last 30 million years
- What does this mean for the current efficiency of C3 photosynthesis?
New pathways evolved ~30 MYA
Evolution of C4 photosynthesis
- Calvin cycle moved to Bundle Sheath Cells surrounding leaf veins
- some chloroplasts also moved inside bundle sheath cells
- Brainstorm: How would mutations/genes be involved?
- BSC cells are less permeable to gases
- what does this mean for photorespiration?
- New ring cell arrangement = Kranz Anatomy
- bundle sheath cells surround veins
- mesophyll cells surround bundle sheath cells
C4 plants concentrate CO2 around Rubisco spatially
- C4 mechanism separates Calvin cycle from high O2
- C4 pathway in mesophyll cell chloroplasts
- C3 pathay in bundle sheath cell chloroplasts
- New enzyme, phosphoenolpyruvate carboxlylase (PEPC)
- fixes CO2 to PEP (no affinity for O2)
- new 4C malate pumped to bundle sheath cells
- malate broken down to release CO2
- Hyper efficient with use of CO2
- How does this impact stomata behavior?
C4 is turbo-charged from of photosynthesis
- Most productive C4 plants have rates ~50% higher than C3 plants
- evolved independently ~70 times
- ~435,000 plant species, only 2% are C4
- 3 of top 10 crop plants are C4
- extremely rare in trees (1 family)
- C4 plants still account for 25% of productivity
Why is C4 photosynthesis not dominant?
- Regeneration of PEP expensive (ATP)
- in addition to ATP needed for Calvin cycle
- Where is needed ATP generated?
- Where should C4 plants live
- efficiency vs dehydration
In hot conditions, the benefits of reduced photorespiration likely exceed the ATP cost of moving CO2 from the mesophyll cell to the bundle-sheath cell
C4 photosynthesis: Evolved independently 60-70 times
C4 vs C3 vs principles of evolution
- Recruitment of enzymes into new functions
- Massive shifts in distribution of parts
- proteins and organelles
- Anatomical modifications to cell structure
- vein structure
- Predict: In what type of plants would this most likely occur?
Why are there virtually no C4 trees?
- Is it difficult for trees to transition from C4→C4?
- long generation times
- Why do C4 shrubs not evolve into C4 trees?
- they should have improved growth…
- Was shade a problem?
- some Hawaiian C4 trees live in understory
- C4 plants seem to have an issue growing tall…
Why C4 matters to you: Water-use-efficiency
- Climate change impacts photosynthesis:
- ↑ CO2
- ↑ temperatures
- variable precipitation
- variations in humidity
- C4 plants can make the same amount of sugars as C3, with less water usage
- more CO2 assimilated per molecule of water loss
- ability to keep stomata closed more often
- C3 & C4 plants should both benefit from extra CO2
- only works in C3 if enough H20
Why C4 matters to you: Crops
- C4 plants are economically important
- corn, sugarcane, sorghum & switchgrass
- So are C3 crops
- beans, rice, wheat, potatoes (temperate crops)
- Global water use is set to triple by 2050
- 70% to the agriculture sector
- 50% transpired through stomata
Can science TUNE photosynthesis?
- Eliminating photorespirtion is unrealistic
- can we tweak Rubisco?
- Small improvement = huge gains in crop yield
- 5% reduction = extra 68 million bushels of soybean
- 23 million bushels of wheat
- $540 million value (Walker et al. 2016)
- 23 million bushels of wheat
Can science TUNE photosynthesis?
Genome work on C4 plants already underway
Evolution of CAM photosynthesis (C4 pathway w/out BSC)
CAM concentrates CO2 around Rubisco temporally
- Stomata open at night only
- 4C compounds produced in large quantities
- stored when Rubisco is inactive
- C4 vs CAM
- substitute BSC for “night shift”
Yes we CAM!!
- CAM evolved independently several times
- ~16,000 species
- Exist in arid environments or habitats
- cacti
- epiphytes
- Are CAM plants fast or slow growing?
C3 vs C4 vs CAM: Know advantages/disadvantages of each
